Solubility of molybdenite (MoS2) in aqueous fluids at 600-800 °C, 200 MPa: A synthetic fluid inclusion study

Solubility experiments of molybdenite in single-phase, NaCl (+/- HCl)-bearing aqueous fluids were conducted at 600-800 degrees C, 200 MPa and various fO(2)-fS(2) conditions imposed by mineral buffers. Small aliquots of fluids were trapped after 1-7 days of equilibration as synthetic fluid inclusions in quartz and subsequently analyzed by laser-ablation ICP MS. Measured Mo concentrations range from 20 to 3000 ppm by weight and increase with increasing temperature, NaCl concentration and oxygen fugacity, but decrease with increasing sulfur fugacity. Our solubility data can be fitted by the following equation: log C-Mo = 0.458 x log fO(2) - 0.463 x log fS(2) + 0.731 x log C-NaCl - 1.57 x (1000/T) + 6.37, where C-Mo and C-NaCl are the concentrations of Mo and NaCl in ppm by weight, fO(2) and fS(2) are fugacities of oxygen and sulfur in bar, and T is temperature in Kelvin. This equation reproduces our data and those of a previous study conducted at 400 degrees C within 0.24 log units, with the exception of two points with excess HCl that are reproduced within <= 0.87 log units. The observed dependencies on fO(2), fS(2), NaCl concentration and pH are compatible with MoS2 dissolution as NaHMoO2S2. If combined with Mo concentrations measured in natural fluid inclusions, our data provide important constraints on the fO(2) and fS(2) conditions of ore formation. While Mo contents of fluids trapped at magmatic conditions agree well with predicted MoS2 solubilities along the magnetite-pyrrhotite fO(2)-fS(2) buffer, Mo concentrations in hydrothermal brines trapped at 400-500 degrees C require orders of magnitude higher fO(2) or lower fS(2) conditions relative to this buffer. The fact that fS(2) in porphyry systems usually increases relative to the magnetite-pyrrhotite buffer during cooling from >700 degrees C to 400 degrees C suggests that the observed high Mo contents are due to high fO(2), which is in agreement with the general trend of increasing fO(2) during subsolidus cooling of granitic plutons. (C) 2011 Elsevier Ltd. All rights reserved.